# Custom boundary condition question

**URL:** <https://festim.discourse.group/t/custom-boundary-condition-question/117>\
**Category:** User Support\
**Created:** [May 26, 2025, 3:29pm UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117 "2025-05-26T15:29:05Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![cruiseraurora](https://avatars.discourse-cdn.com/v4/letter/c/ecd19e/32.png) [@cruiseraurora](https://festim.discourse.group/u/cruiseraurora)\
**Post date:** [May 26, 2025, 3:29pm UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/1 "2025-05-26T15:29:05Z")

</div>

Hello! Is it possible to set the boundary condition as a function of time in festim? I want to modelling TDS-spectra with that condition: if time \>= 100 (for example), boundary condition set to RecombinationFlux and if time \< 100, boundary condition set to DirichletBC. I tried to use CustomFlux in that way:

```auto
def Kr_t(test_time):
    Kr_0 = 1e-28,
    Kr_E = -0.27,
    if test_time >= 100:
        Flux_t = -Kr_0*sp.exp(-Kr_E/8.625e-5/my_model.T)*(F.solute)^2, #recombination
    else:
        Flux_t = 0 #Dirichlet with value = 0
    return Flux_t
my_model.boundary_conditions = F.CustomFlux(
    field = 0,
    surfaces = [1],
    function = Kr_t,
    test_time = my_model.t,
)

```

but it gives me an error “stepsize reached minimal value”. If I set BC to only RecombinationFlux(1e-28, -0.27, 2, surfaces = [1]) or DirichletBC(surfaces=[1], value=0, field=0), it works.

---

<div class="post-metadata">

**Author:** ![VVKulagin](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/vvkulagin/32/51_2.png) [@VVKulagin](https://festim.discourse.group/u/VVKulagin)\
**Post date:** [May 26, 2025, 3:53pm UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/2 "2025-05-26T15:53:24Z")

</div>

Hi @cruiseraurora!

> Is it possible to set the boundary condition as a function of time in festim?

Yes, it is. You can do it with the class in your example, but notice that the first 2 arguments for `F.CustomFlux` are `T` and `solute` fields at the boundary, as stated [here](https://festim.readthedocs.io/en/latest/_modules/festim/boundary_conditions/fluxes/flux_custom.html#CustomFlux). The API docs also show an example of usage:

```auto
def fun(T, solute, param1):
    return 2*T + solute - param1

my_bc = CustomFlux(
    surfaces=[1, 2],
    function=fun,
    field=0,
    param1=2*festim.x + festim.t
)

```

I’m not 100% confident that if-clause would work properly if you want to define a piece-wise function. Probably, you might need to use `sp.Piecewise` or `fenics.conditional`, but it requires testing.

---

<div class="post-metadata">

**Author:** ![cruiseraurora](https://avatars.discourse-cdn.com/v4/letter/c/ecd19e/32.png) [@cruiseraurora](https://festim.discourse.group/u/cruiseraurora)\
**Post date:** [May 27, 2025, 10:59am UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/3 "2025-05-27T10:59:28Z")

</div>

You mean that instead of `def Kr_t(test_time)` I should try `def Kr_t(T,solute,test_time)`?

Now my code looks like this:

```auto
def Kr_t(T,solute,test_time):
    Kr_0 = 1e-28,
    Kr_E = -0.27,
    Flux_t = sp.Piecewise(
        (-Kr_0*sp.exp(-Kr_E/8.625e-5/T)*(solute)^2, test_time >= implantation_time+cold_time),
        (0, True))
    return Flux_t
my_model.boundary_conditions = F.CustomFlux(
    field = 0,
    surfaces = [1],
    function = Kr_t,
    test_time = my_model.t,
)

```

but I still have an error “stepsize reached minimal value”.

---

<div class="post-metadata">

**Author:** ![VVKulagin](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/vvkulagin/32/51_2.png) [@VVKulagin](https://festim.discourse.group/u/VVKulagin)\
**Post date:** [May 27, 2025, 11:25am UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/4 "2025-05-27T11:25:10Z")

</div>

Yeah, I meant to use the custom flux BC as you showed, but exponentiation should be made with `**` rather than the XOR operator `^`.

Additionally, it’s difficult to suggest anything as you don’t provide the details about when the error appears (at the start or during the computation), what are the stepsize function, mesh, etc.

---

<div class="post-metadata">

**Author:** ![remidm](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/remidm/32/4_2.png) [@remidm](https://festim.discourse.group/u/remidm)\
**Post date:** [May 27, 2025, 12:35pm UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/5 "2025-05-27T12:35:08Z")

</div>

@cruiseraurora if I understand correctly you don’t want to change the value of the flux but rather change the type of the BC in the middle of the simulation, from a recombination BC (Robin type) to a fixed concentration of zero (homogeneous Dirichlet BC). Correct?

This specific case is not trivial in FESTIM. Here are your options:

- split your problem in two simulations: the first with the RecombinationFlux, the second simulation with the Dirichlet BC
- instead of changing the BC type, consider artificially increasing the Kr value so that the concentration is almost zero (ie. instantaneous recombination)

As of the exact reason of the `stepsize reached minimal value` error, this is due to your problem not converging and it hard to debug without a minimal working example (MWE) reproducing the issue

---

<div class="post-metadata">

**Author:** ![cruiseraurora](https://avatars.discourse-cdn.com/v4/letter/c/ecd19e/32.png) [@cruiseraurora](https://festim.discourse.group/u/cruiseraurora)\
**Post date:** [May 27, 2025, 12:35pm UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/6 "2025-05-27T12:35:09Z")

</div>

The error appears after 1.7e1 sec - console writes this  
`0.0 % 1.7e+01 s Elapsed time so far: 11.7`  
the computation slows donw and after a few seconds it prints this

```auto
Traceback (most recent call last):time so far: 13.9 s
  File "/home/wsluser/FESTIMfolder/Rusferplasma1e25.py", line 166, in <module>
    my_model.run()
  File "/home/wsluser/anaconda3/envs/festim-env/lib/python3.11/site-packages/festim/generic_simulation.py", line 422, in run
    self.run_transient()
  File "/home/wsluser/anaconda3/envs/festim-env/lib/python3.11/site-packages/festim/generic_simulation.py", line 445, in run_transient
    self.iterate()
  File "/home/wsluser/anaconda3/envs/festim-env/lib/python3.11/site-packages/festim/generic_simulation.py", line 473, in iterate
    self.h_transport_problem.update(self.t, self.dt)
  File "/home/wsluser/anaconda3/envs/festim-env/lib/python3.11/site-packages/festim/h_transport_problem.py", line 338, in update
    dt.adapt(t, nb_it, converged)
  File "/home/wsluser/anaconda3/envs/festim-env/lib/python3.11/site-packages/festim/stepsize.py", line 101, in adapt
    raise ValueError("stepsize reached minimal value")
ValueError: stepsize reached minimal value

```

I try to simplify CustomFlux as much as possible and make it look like DirichletBC:

```auto
def Kr_t(T,solute,test_time):
    Flux_t = 0
    return Flux_t
my_model.boundary_conditions = F.CustomFlux(
    field = 0,
    surfaces = [1],
    function = Kr_t,
)

```

but it gives me the same error (“stepsize reached minimal value”) at the same time (1.7e1 sec). If I change BC from CustomFlux to DirichletBC with arguments `(surfaces=[1], value=0, field=0)`, DirichletBC causes no troubles in my computations. Why? I really apologize for my misunderstanding of this situation…

---

<div class="post-metadata">

**Author:** ![remidm](https://yyz2.discourse-cdn.com/free1/user_avatar/festim.discourse.group/remidm/32/4_2.png) [@remidm](https://festim.discourse.group/u/remidm)\
**Post date:** [May 27, 2025, 12:37pm UTC](https://festim.discourse.group/t/custom-boundary-condition-question/117/7 "2025-05-27T12:37:30Z")

</div>

@cruiseraurora careful here, setting the flux to zero is very different from setting the concentration to zero.

Constraining the particle flux (Flux BC) enforces the -D \nabla c \cdot n value at the boundary whereas a DirichletBC enforces the c value at the boundary